Several L-Band global and area mobile satellite communications system are presently in operation and others are in an advanced planning stage. A system for satellite communication with passenger aircraft is also about to go into operation.
The aeronautical satellite communications system (Avsatcom) enables aircraft to satellite communication in the L Band and it provides a satellite to shore link in the C Band. For satisfactory operation in these bands, however, a diplexer must satisfy stringent specifications. For example, typical Inmarsat Specifications are shown below in Tables 1A and 1B and these may be compared with the more recent requirements shown in Table 2.
TABLE 1A ______________________________________ TYPICAL INMARSAT STANDARD A DIPLEXER SPECIFICATION - PARAMETER RECEIVE TRANSMIT ______________________________________ Frequency Range 1535-1543.5 1636.5-1645.0 MHz User Bandwidth 8.5 8.5 MHz Rejection at other 120 95 dB channel Insertion Loss 0.7 0.7 dB Transmit Power 50-100 W Ratio channel separation/ 11.7 bandwidth ______________________________________
TABLE 1B ______________________________________ TYPICAL INMARSAT STANDARD B DIPLEXER SPECIFICATION PARAMETER RECEIVE TRANSMIT ______________________________________ Frequency Range 1530-1545.0 1625.5-1646.5 MHz User Bandwidth 15.0 20.0 MHz Rejection at other 120 95 dB channel Rejection to 4 GHz 70 70 dB Rejection 4 to 20 GHz 50 50 dB Insertion Loss 0.7 0.7 dB Transmit Power 50-100 W Ratio channel separation/ 5 bandwidth ______________________________________
TABLE 2 ______________________________________ AERONAUTICAL DIPLEXR OUTLINE SPECIFICATION PARAMETER RECEIVE TRANSMIT ______________________________________ Frequency Range 1530-1559 1626.5-1660.5 MHz Bandwidth 29 34 MHz Return Loss 20 20 dB Insertion Loss 08 0.8 dB Rejection at other 120 120 dB channel Rejection 1565 to 1585 100 dB Transmit Power 40-100 W Ratio channel separation/ 2.91 bandwidth ______________________________________
Whilst Inmarsat Standard A requirements may be met with two chebycheff filters as used in conventional diplexer, the next generation of Inmarsat spacecraft at present under construction require larger bandwidth as set out by the major specification points outlined in Table 2.
As can be seen, a lower separation to bandwidth is now required for the transmit filter as well as over 100 dB of rejection relatively close to the transmission passband. This is to protect nearby GBF receivers on an aircraft.